Eruptive History of Three Fingered Jack, a Mafic Composite Cone in the Central High Cascades of Oregon

Javaria Aziz

Portland State University

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The central High Cascades graben has been the center of mafic volcanism with approximately 60 km³ of lava erupting from scoria cones and mafic composite cones in the past 15,000 years. Three Fingered Jack is one of these mafic composite cones, located south of Mount Jefferson, near the town of Sisters, Oregon. Three Fingered Jack is composed of basaltic andesite lava flows, scoria and pyroclastic flows, that are cut by dikes fed by an intrusive plug. This talk will discuss the eruptive product of Three Fingered Jack and the magmatic processes involved in the production of these lavas. 

Please join us at:

Worthy Brewing (eastside location)

In the Hop Mahal Room

5:30 pm social hour

7:00 pm presentation

COGS talk are free and open to the public -- all are welcome! Please join us for the social hour before the presentation.

All presentations are also live-streamed through Zoom. There will be a registration link at the top of this page as we get closer to the date of this presentation.